This invention relates to a process for preparing ultraphosphoric and polyphosphoric acid. This invention also relates to a process for preparing such phosphoric acids while simultaneously recovering valuable energy from burning of phosphorus.
Polyphosphoric acid, sometimes referred to as condensed phosphoric acid, is a mixture of polymerized acids having a general formula of H.sub.n+2 (P.sub.n O.sub.3n+1). It is used in the manufacture of polymerization catalysts, such as the production of propylene tetramer and for cyclizations, acylations and esterifications in organic syntheses. Polyurethane foams are also flame proofed with polyphosphoric acid. Polyphosphoric acid is produced in commercial quantities to satisfy these needs.
The prior art discloses several different ways of preparing polyphosphoric acid. One method involves mixing phosphorus pentoxide, with an appropriate amount of water in a stirred, cooled chamber. A second method involves heating mixtures of phosphorus pentoxide with orthophosphoric acid. A third method involves restricting the amount of water in certain types of phosphoric acid plants to obtain the desired polyphosphoric acid. Still another method involves boiling orthophosphoric acid to evaporate water until the desired concentration is reached. The maximum concentration that can be reached by the latter method is an azeotropic mixture which varies from 91.1 to 92.2 percent P.sub.2 O.sub.5 depending upon the system pressure; however, the boiling point and corrosion increase rapidly as the acid becomes more concentrated and effectively limits the concentration to well below the azeotrope.
As is known to those skilled in the art, the phosphorus pentoxide useful to make polyphosphoric acid is prepared commercially by burning elemental phosphorus in a stream of dried air, allowing adequate time for completion of the reaction to prevent the formation of lower oxides of phosphorus. The resulting phosphorus pentoxide can be made so free of lower oxides that it will not decolorize dilute permanganate solutions. The usual plant for manufacturing phosphorus pentoxide consists of a phosphorus feed system, provisions for drying the air, a burning chamber, and a process step in which the phosphorus pentoxide gas is condensed and/or contacted with water to make phosphoric acid.
The prior art on the preparation of phosphorus pentoxide from elemental phosphorus abounds with references to the disastrous corrosive attack on metals of construction at elevated temperatures unless the system was kept very dry to avoid the formation of polymeric phosphoric acid. The condensation of polymeric acids on hot metal surfaces was thought to be particularly corrosive. The polymeric acids were commonly called "meta acid", a term used loosely to describe any polymeric acid composition that is not fluid at room temperature. Such compositions are those above about 85 percent P.sub.2 O.sub.5. To avoid this corrosive attack, metal surfaces were either protected by a layer of refractory or maintained below 100.degree. C. where the corrosion rate on Type 316 stainless steel is tolerable, and this method is used in most phosphoric acid plants capable of producing polyphosphoric acid from elemental phosphorus. Neither of these methods are attractive for recovery of heat at useful steam distribution pressures.
Although satisfactory results are achieved by using the prior art processes for preparing phosphorus pentoxide and thereafter converting the phosphorus pentoxide to polyphosphoric acid, there are certain disadvantageous aspects to the prior art processes. For one thing, no practical method has been developed for recovering the enormous amount of heat that is evolved from burning elemental phosphorus in air. Indeed, some prior art processes for preparing polyphosphoric acid are very energy intensive. All of the prior art processes require careful control of the conditions in order to obtain a satisfactory product and to avoid corrosion, and in all cases it is difficult to obtain a product with a high concentration of P.sub.2 O.sub.5.
Now, according to the process of the present invention, ultraphosphoric acid can be prepared in the phosphorus burner while simultaneously recovering much of the heat that is generated by burning the elemental phosphorus, and such heat can be used for other processes rather than generate the heat by conventional means. The ultraphosphoric acid thus produced is a valuable intermediate in the preparation of polyphosphoric acid, and the concentration of the ultraphosphoric acid is limited only by its melting point.